The present invention relates to a process for catalytically hydrotreating heavy hydrocarbon fractions and in particular to alumina catalysts used in such a process.
Porous alumina has often been employed as a support material for various hydroprocessing catalysts for hydrocarbonaceous materials. When the feedstock is more or less free of soluble metal impurities, such as iron, nickel, vanadium and the like, ordinary porous aluminum is generally satisfactory as catalyst support. If, however, the feedstock contains high concentrations of soluble metal impurities, a conventional aluminum processing catalyst tends to rapidly build up metals near the pore mouth, thereby plugging the pores of the catalyst and reducing its activity. Since regeneration of a catalyst by removal of metals is generally impractical, the life time of a conventional alumina catalyst is limited to the time it takes to plug the pores.
Because of the shrinking world supply of oils, oil processors are faced with the necessity of utilizing feedstocks that are highly contaminated with metals. In the processing of these feedstocks, it is very desirable to remove as much of the contaminating metals as early in the refining of these feedstocks as possible, so that other downstream hydroprocessing catalysts do not suffer build-up of metals and consequent reduced activity. Removing metals also makes a higher quality final product that is less corrosive and less polluting when combusted.
A variety of demetalation catalysts have been proposed, including many that are supported by a porous refractory inorganic oxide, such as alumina. U.S. Pat. No. 3,898,155 describes a catalyst which has a wide range of pore sizes including a specified percentage of pore sizes greater than 600 Angstroms and an average pore size greater than 100 Angstroms that has high catalytic metals content. U.S. Pat. No. 4,257,922 describes a hydroprocessing catalyst for hydrocarbonaceous material derived from coal that has a bimodal pore distribution and high metals content. U.S. Pat. No. 4,119,531 describes a catalyst having pores of an average diameter of at least 125 Angstroms but no mention is made of the desirability of a substantial portion of the pore volume coming from pores of large diameter.